Wind power has received increased attention as being one of the cleanest, most environmentally friendly energy sources presently available. A typical modern wind turbine can include a tower, a generator, a gearbox, a nacelle, and a rotor having one or more rotor blades. The rotor blades can transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The one or more generators can be, for instance, coupled to the rotor via a gearbox. The gearbox can step up the inherently low rotational speed of the rotor such that the generator can efficiently convert the mechanical rotational energy to electrical energy, which can be fed into a utility grid via at least one electrical connection.
Wind turbines can use a variable speed operation such that the speed of a turbine blade changes with changes in wind speed. However, as the speed of the turbine fluctuates, the frequency of alternating current flowing from the generator also fluctuates. Accordingly, variable speed turbine configurations can also include power converters that can be used to convert a frequency of generated electrical power to a frequency substantially similar to a utility grid frequency. Such power converters can typically comprise an AC-DC-AC topology with a regulated DC link, and can be controlled by a converter controller.
Such wind turbines can use variable speed operations to optimize loads and to improve turbine output. In particular, wind turbines are most efficient when they operate at an optimum tip-speed ratio. Tip-speed ratio is the ratio between the tangential speed of the tip of a turbine blade and the velocity of the wind at the wind turbine. Accordingly, a wind turbine will collect more wind energy operating at the optimum tip-speed ratio than it will if operating outside the optimum tip-speed ratio.
For many wind turbines, the operating space, and hence value to the customer, is limited by maximum voltages for one or more wind turbine components inherent to wind turbine systems. For instance, a power converter in a wind turbine system can have a voltage constraint that limits the minimum and maximum speed values of the generator.
Thus, a need exists for systems and methods for increasing the power output of a wind turbine system at reduced wind speeds while also maintaining power converter voltage levels within specified operating limits.